The demand for higher data rates and performance is constantly increasing in communications. One approach that is currently used for meeting this increased demand is the implementation of multiple-input, multiple-output (MIMO) antenna systems.
In MIMO systems, data streams are individually mapped and modulated prior to being pre-coded and transmitted by different (physical) antennas of a transmitter. The combined data streams are received by multiple antennas of a receiver. The receiver extracts the separate data streams from the combined data streams by using, for example, minimum mean-square error (MMSE) or successive interference cancellation (SIC) algorithms.
Downlink (DL) single-user (SU) MIMO was introduced in LTE in release-8. Uplink (UL) SU-MIMO has recently been standardized in 3GPP for release-10. Both DL and UL SU-MIMO use identical same-layer mapping schemes as illustrated in FIG. 1 to map symbols from different codewords to the layers used for transmission.
As illustrated in the layer-to-codeword mapping (FIG. 1), in some cases, one codeword may be mapped to multiple layers (such as in rows 3, 4 and 5 where the number of codewords is less than the number of layers).
UL LTE employs single-carrier frequency-division multiple-access (FD-FDMA), which suffers inter-symbol interference (ISI) in multipath dispersive channels. A linear MMSE frequency-domain (FD) equalization is a common solution for UL LTE in alleviating ISI. While this equalization performs well in single-input multiple-output (SIMO) systems, its performance is less than optimal in MIMO systems when MIMO interference, e.g. inter-layer interference is present in addition to ISI.
A successive interference cancellation (SIC) receiver provides better (than MMSE) performance for MIMO reception. SIC receivers with per layer rate control have achieved open-loop MIMO capacity in flat channels as described in “Optimum decision feedback multiuser equalization with successive decoding achieves the total capacity of the Gaussian multiple-access channel” (by M. K. Varanasi and T. Guess, Proc. Asilomar Conf. on Signals, Systems, and Computers, Monterey, Calif., November 1997, pp. 1405-1409), the subject matter of which is incorporated in its entirety herein by reference.
A SIC receiver detects a signal sent by the first layer. Upon detection and decoding of the signal from the first layer, the receiver cancels the interference contributed by the detected first layer signal before it detects the next layer signal. The process is repeated until the signals from all the layers are detected. In typical operation, a layer signal is cancelled when the CRC checks. With perfect per-layer rate-control, the transmission rate for each layer can be chosen to have an arbitrarily low block error rate (BLER). As such, the SIC receiver can always cancel interference contributed by previously detected layers.
In practice, however, certain limitations exist with SIC receivers. The performance of a SIC receiver is largely degraded due to link adaptation inaccuracy. Link adaptation inaccuracy may result in the transmission rate of a layer being higher than what can be supported. In this case, the CRC for this layer signal does not check. Consequentially, the layers that are subsequently detected will have block errors as their rates were determined based on no interference from earlier detected layers. Further, some of the MIMO configurations do not use per layer rate control. When multiple layers are mapped to the same codeword for example, they share the same transmission rate, and thus a deviation from per-layer rate control. In this case, a SIC receiver does not reduce interference contributed by other layers mapped to the same codeword.
What is desired, therefore, is a receiver having improved MIMO reception when multiple layers are mapped to the same codeword.